Owing to its almost unlimited resources and harmlessness, carbon is an excellent material in view of resources and environmental problems. Carbon materials have diverse forms of interatomic bonding, and a variety of crystal structures are known, such as diamond, diamond-like carbon, graphite, fullerene and carbon nanotubes. Above all, diamond-like carbon (amorphous carbon) having an amorphous structure is expected to be applied in each industrial field, because of its high mechanical strength and superior chemical stability.
However, general amorphous carbon films have an electric resistance in the range of semiconductors to insulators. In order to further widen the use of amorphous carbon, it has been requested to impart electric conductivity to amorphous carbon. One use of amorphous carbon is a fuel cell separator. FIG. 8 schematically shows one example of a single-cell solid polymer fuel cell. The left figure of FIG. 8 shows arrangement of its respective constituent elements before lamination, and the right figure of FIG. 8 shows a laminated state of those elements. A single cell 1 is constituted by an electrolyte membrane 1a, and a pair of electrodes (an air electrode 1b and a fuel electrode 1c) sandwiching the electrolyte membrane 1a from both sides. Separators 2 having channel-formed surfaces 2b, 2c on which a plurality of channels are formed. The separators 2 are respectively set in resin separator frames 3 and laminated so that the air electrode 1b and the channel-formed surface 2b face each other and the fuel electrode 1c and the channel-formed surface 2c face each other. Thus, gas passages sectioned by electrode surfaces and the channels are formed between the electrodes and the separators, and fuel gas and oxygen gas, which are reaction gases of the fuel cell, are efficiently supplied to the electrode surfaces.
In the fuel cell, the fuel gas and the oxygen gas need to be separately supplied to the entire electrode surfaces without being mixed. Therefore, the separators need to be gas tight. Furthermore, the separators need to collect electrons generated by a reaction and to have a good conductivity as electric connectors for connecting adjoining single cells when a plurality of cells are laminated. Moreover, because electrolyte surfaces are strongly acidic, the separators are demanded to have corrosion resistance.
Therefore, as a separator material, it is common to use graphite plates. However, because the graphite plates split easily, the graphite plates have a problem in workability in producing separators by forming a plurality of gas passages, making the surfaces flat and so on. On the other hand, because metallic materials are superior in both conductivity and workability and especially titanium and stainless steel have superior corrosion resistance, the metallic materials can be used as separator materials. However, since metallic materials having superior corrosion resistance are easily passivated, there is a problem of increasing internal resistance of a fuel cell and causing a voltage drop. Consequently, a separator in which a surface of a metallic substrate is covered with a conductive amorphous carbon film starts to attract attention.
Examples of a process for imparting conductivity to amorphous carbon include a process of adding a metal to amorphous carbon (see Japanese Unexamined Patent Publication No. 2002-38268 (Patent Document 1) and Japanese Unexamined Patent Publication No. 2004-284915 (Patent Document 2)). However, the added metal may become a cause of corrosion and the metal-added amorphous carbon when used in contact with other metals may become a cause of adhesion, so chemical stability which amorphous carbon inherently has is sometimes damaged.
On the other hand, in Patent Document 3, conductivity is imparted to amorphous carbon without adding metal. Patent Document 3 discloses a carbon film having a structure in which an sp2-bonded crystal having sp2-bonded carbon within part of the crystal extends continuously from the lowermost layer (a substrate side) to the uppermost layer (a surface side) of the film in a film thickness direction and other portions than the sp2-bonded crystal are amorphous. According to the description of Patent Document 3, it is important in view of corrosion resistance and wear resistance of the carbon film to reduce the content of sp2-bonded crystal. Therefore, it is desirable that the sp2-bonded crystal exists continuously from the substrate side to the surface side of the carbon film, because it is effective in increasing conductivity in a film thickness direction of the carbon film and as a result contributes to reducing the content of sp2-bonded crystal. The cited document 3 also states that an increase in the content of the sp2-bonded crystal causes a decrease in hardness and a decrease in wear resistance of the carbon film. That is to say, the cited document 3 suggests that an amorphous portion of the carbon film contains a large amount of sp3-bonded carbon, which improves wear resistance and hardness. As a result, there is a possibility that conductivity in a perpendicular direction to the film thickness direction in which there is no continuous sp2-bonded crystal deteriorates and that electrical anisotropy occurs.
Moreover, Japanese Unexamined Patent Publication No. 2000-67881 (Patent Document 4) and Japanese Unexamined Patent Publication No. 2005-93172 (Patent Document 5) disclose a fuel cell separator in which an amorphous carbon film mainly comprising carbon and hydrogen is formed on a surface of a metallic material, using methane as a raw material. However, it is assumed that the separators of Patent Document 4 and Patent Document 5 having an amorphous carbon film formed by an ordinary film-forming process using methane as a raw material do not have sufficient conductivity or corrosion resistance required for separators. Especially in Patent Document 4 and Patent Document 5, characteristics were not evaluated under severe conditions where a voltage expected in actual environment was applied to the separators, so it is believed that the separators when used in fuel cells do not exhibit required conductivity or corrosion resistance.